Childhood Vaccination Coverage in the U.S.-Affiliated Pacific Islands, 2017–2021 Births

Childhood vaccination is one of the most successful public health measures there is: it lengthens lives, lowers health care costs and slows the spread of preventable disease. All six U.S.-affiliated Pacific Islands (USAPI) take part in the U.S. domestic immunization program, and their immunization programs work with CDC to track coverage. This CDC report, the first comprehensive analysis of coverage trends in the islands, looked at children born from 2017 through 2021 in five of the six jurisdictions: American Samoa, the Northern Mariana Islands, the Federated States of Micronesia, the Marshall Islands and Palau.

The goal

Healthy People 2030 and the World Health Organization's Immunization Agenda 2030 both set a target of at least 90% coverage by age 2 with one or more doses of measles, mumps and rubella (MMR) vaccine and four or more doses of diphtheria, tetanus and acellular pertussis (DTaP) vaccine. WHO's agenda adds three or more doses of pneumococcal conjugate vaccine (PCV).

How coverage was measured

Records came from each jurisdiction's immunization information system. Children were grouped by year of birth. Those listed as inactive or deceased, and those with no recorded doses, were left out to avoid inflating the count. On-time coverage counted doses received by the day before a child turned 24 months (rotavirus vaccine by 8 months, because of its upper age limit). To measure catching up, the analysts also counted every dose received by June 1, 2024.

Coverage by age 2

The 90% mark was reached for at least one birth cohort in these cases:

• three or more doses of DTaP, polio vaccine and hepatitis B vaccine in the Northern Mariana Islands and Palau;
• one or more doses of MMR and the Hib primary series in the Northern Mariana Islands, the Federated States of Micronesia and Palau;
• the hepatitis B birth dose in American Samoa, the Northern Mariana Islands, the Marshall Islands and Palau; and
• three or more doses of PCV in Palau.

It was not reached anywhere for four or more doses of DTaP, the full Hib series, four or more doses of PCV, or rotavirus vaccine. Coverage with the combined six-vaccine series was below 75% in every jurisdiction. Hepatitis A and varicella coverage in the two U.S. territories, and two-dose MMR coverage in the three freely associated states, were all below 90%.

Ups and downs by birth year

Coverage swung from one birth cohort to the next. For MMR, it ranged from 68.2% to 91.6% in the Federated States of Micronesia and from 87.4% to 96.6% in Palau; for four doses of DTaP, from 39.6% to 60.6% in the Federated States of Micronesia and from 73.4% to 85.4% in Palau.

In general, children born in 2018 were better covered than those born in 2017, and those born in 2019 and 2020 were less well covered than the 2018 cohort. Among children born in 2021, coverage with some vaccines began to rise again — for MMR, by 5.5 percentage points in American Samoa, 16.5 in the Federated States of Micronesia and 9.2 in Palau compared with the 2020 cohort — though the pattern differed by vaccine and place.

Catching up later

By June 1, 2024, coverage had risen for every vaccine, in every jurisdiction and cohort, showing that many children were vaccinated after age 2. The largest gains were for four doses of DTaP, 9.2 to 20.7 percentage points; by June 2024 that coverage ranged from 74.0% to 84.4% across birth cohorts in American Samoa and from 91.6% to 94.8% in Palau. For children born in 2021, several vaccines crossed from below 90% to 90% or more, including one dose of MMR in American Samoa, the Federated States of Micronesia and the Marshall Islands.

CDC chart: coverage by age 24 months and catch-up coverage by June 2024, children born in 2021.

Why coverage lags

Coverage in the islands was substantially below U.S. national estimates. Program staff cite a lack of reliable transportation to take vaccine to remote communities, a shortage of expertise for data-driven outreach plans, and problems with governance and timely release of funding. Differences in funding between jurisdictions may matter too, but research on each jurisdiction's specific barriers is lacking. The COVID-19 pandemic may have made things worse: children who turned 2 in 2020 were better covered than those who turned 2 afterward, as already thin staff were stretched further.

What could help

Catch-up campaigns are recommended for the children born in 2017–2021, but models suggest that reliable routine vaccination services are the more cost-effective long-term approach. Proven methods include strong recommendations from health care providers, promoting vaccines at every opportunity, and reminder and recall notices. For remote populations, more frequent vaccine delivery to outer islands should be considered, and lessons from successful campaigns such as the COVID-19 vaccine rollout could be applied to routine services. Interviews with immunization program stakeholders could help identify what helps and hinders each program.

Limitations

• Estimates are only as complete and accurate as the island registries, though checks since 2016 have found them highly complete and accurate.
• Registries can overcount active patients because people who move away or die are hard to track, which would understate coverage.
• Guam's coverage is measured by a different survey, so it was not included and is not directly comparable.

Sources

• Ashley Tippins, E.M. Boyd, Kelsey C. Coy, Glodi Mutamba and Jennifer L. Kriss, "Vaccination Coverage by Age 24 Months Among Children Born During 2017–2021 — U.S.-Affiliated Pacific Islands," Morbidity and Mortality Weekly Report, CDC: https://www.cdc.gov/mmwr/volumes/73/wr/mm7338a4.htm